1. Field of the Invention
This invention relates to a burner, particularly to one for burning a gaseous fuel, and further relates to a method of burning a gaseous fuel in a manner to produce combustion gases having an ultra low content of nitrogen oxide. Hereinafter, nitrogen oxides, which are primarily nitric oxide and nitrogen dioxide, are collectively referred to as "NOx".
Major environmental and other problems have been encountered in the production of flue gases containing high contents of NOx. The NOx tends to react under atmospheric conditions to form environmentally unacceptable conditions, including the widely known phenomena known as urban smog and acid rain. In the United States and elsewhere, environmental legislations and restrictions have been enacted, and more are expected to be enacted in the future, severely limiting the content of NOx in flue gases.
In U.S. Pat. No. 4,874,310, granted Oct. 17, 1989 to Selas Corporation of America, the assignee hereof, a controlled primary air inspiration gas burner was disclosed, in which the introduction of control primary air was controlled in order to provide a substantial reduction of the content of nitrogen oxides in the flue gas. Such a burner includes extra piping for the introduction and control of the primary air, and this sometimes introduces expense and possible complications, especially in furnace installations utilizing a very large number of burners.
In U.S. Pat. No. 5,044,931, granted Sep. 3, 1991, Selas Corporation of America, the assignee hereof, was granted a patent for an apparatus comprising a burner which not only reduces the NOx content in the combustion gases but also radically increases the burner capacity even for the same size burner. In the burner described in this patent, secondary gaseous fuel is being burned in addition to primary gaseous fuel, and the premix introduced through the combustion passageways forms a screen of burned gases which dilute the admixture of secondary gas and secondary air, slowing the secondary fuel reaction rate. However, there are economic limitations as to the cost required in manufacturing and installing burners of the type described in the aforesaid U.S. Pat. No. 5,044,931, and it is an objective of this invention to provide a burner having a simpler and less expensive construction from the manufacturing point of view, one having a small inspirator and one less tip, which does not require expensive machining and which is easy to drill and to assemble.
Other endeavors have been made to reduce the content of NOx in furnace flue gases but many have been found unattractive in view of their requirement of too much operator attention, and in view of the need for extremely attentive control in order to assure that there will be no violation of existing environmental laws. It is very important to be able to obtain a very substantial reduction of NOx content so that even in the event of operator error the environmental law will not be violated and the further operation of the plant and its equipment will not be enjoined by governmental action.
Prior to the discovery forming the basis of the Selas U.S. Pat. No. 5,044,931, it was the general indication in the prior art for premix burners that reduced NOx contents can be obtained by avoiding secondary air, by using substantially entirely primary air, and by firing the burner as close as possible to its maximum firing capacity. The foresaid Selas patent disclosed for the first time that a premix could form a screen of burner gases which dilute the admixture of secondary gas and secondary air, slowing the secondary fuel reaction rate.